The present invention relates in general to relative humidity sensors and, more particularly, the invention relates to sensors made from cobalt oxide as the active element.
Procedures for manufacturing cobalt oxide humidity sensors or hygrometers are well known. The starting material is a cobalt oxide powder. As pure CoO powder is very expensive, the starting material is generally a mixture of CoO with some Co.sub.3 O.sub.4, but the latter compound dissociates at about 900.degree.C. so the completed sensor will be essentially CoO, the cobaltous oxide. This compound is stable up to its melting point, which is above 1800.degree.C.
The finely divided powder, preferably minus 325 mesh, is mixed with an inert liquid vehicle and viscosifiers to form a screen-printable paste. A thin layer of the paste is then screen-printed in a desired pattern onto a dielectric, high-temperature resistant substrate, typically a high-alumina ceramic. The screened pattern is then dried and fired in air at a temperature in the range of 1350.degree.C. to 1550.degree.C. Electrodes can be performed on the substrate, co-fired with the paste, or applied in a subsequent operation. The latter is the more common approach, as it is generally desired to have the electrodes in a rather elaborate interdigitated pattern on the top surface. Conductive inks or pastes are used in the conventional manner.
Before such a sensor can be put to use, it must be accurately calibrated to determine the change in electrical resistance with relative humidity.